As an insert-exchangeable, rotary tool suitable for high feed, Japanese Patent Laid-Open No. 2000-126920 discloses a face-milling cutter equipped with circular throwaway inserts. This face-milling cutter comprises round throwaway inserts each having a mounting hole at a center, cylindrical holders each having at a tip end a supporting seat for fixing the round throwaway insert by a flat head screw in such a direction that its upper surface serves as a flank and its side surface serves as a face, a cutter body for fixing the cylindrical holders, and wedges each inserted into a plurality of circular holes provided in a peripheral portion of a front surface of the cutter body in parallel with its axis and pressed into a gap between a flat surface provided on a periphery of each cylindrical holder and the cutter body. When fixed to the holder, the throwaway insert is inclined toward the rotational rearward direction and the periphery of the cutter, with a cutting edge formed by a crossing edge of the upper surface and the side surface being outward curved in both front surface direction and rotation direction of the cutter. Thus, with each round throwaway insert attached to the tool inclined in two directions, a large-radius, circular cutting edge is provided, providing reduced impact at high-feed cutting.
Because this face-milling tool comprises round throwaway inserts, the surfaces of each insert with which the insert is fixed to the tool are a bottom surface and a side surface. However, because the side surface of the insert is a curved surface, precise positioning is difficult, needing an edge position-controlling screw. Further, because the round insert is attached inclined in two directions, the insert is likely to rotate during cutting, resulting in the likelihood that the edge position deviates. Accordingly, this tool is suitable for working at a cutting depth of as small as 0.1-1 mm or less, and if the tool is designed bigger for deeper cutting, an edge engaged to cutting becomes too long, resulting in too large cutting resistance.
As another example of high-efficiency, insert-exchangeable, rotary tools, Japanese Patent Laid-Open No. 2000-5921 discloses, as shown in FIG. 24, a high-feed throwaway-type rotary tool comprising a plurality of throwaway inserts 301, each throwaway insert 301 having a straight or circular peripheral cutting edge so as to have a back taper δ to a circular major cutting edge 311 and a center axis of tool 302, and the major cutting edge 311 having a cutting edge angle κ of 3-35°. With the circular major cutting edge 311 of the throwaway insert 301, chips generated at the initial stage of cutting are thin and become gradually thicker as the tool rotates. Accordingly, a large impact force is not applied to the tool, enabling high feed in the cutting of steel suffering from little variation of cutting conditions. For instance, in the case of cutting steel, which is free from intermittent cutting, stable cutting is achieved even under the conditions of a feed fz per one tooth of 2 mm/tooth.
It has been found, however, that this high-feed, throwaway-type, rotary tool is not necessarily satisfactory to achieve a higher feed per one tooth. In addition, because this throwaway insert 301 has circular major cutting edges 311 on two opposing surfaces with two other flat opposing surfaces for abutting the tool, it is not economically advantageous.
Accordingly desirable are a long-life, insert-exchangeable, rotary tool such as a face-milling tool capable of performing stable cutting without cracking throwaway inserts even for high-feed working at a cutting depth of about 1-6 mm, particularly for works suffering from large variations of cutting conditions such as works having many holes and pre-hardened steel, etc., and throwaway inserts used therefor.